1. Field of the Invention
The present invention relates to methods and apparatus utilized in gingival tissue management during the preparation of artificial crowns and bridges. More particularly, the present invention is directed to construction and use of gingival retraction cord for retracting gingival tissue preparatory to making an impression of a tooth that has been prepared for fitting of an artificial crown.
2. The Prior Art
It has become a common dental procedure to replace a damaged or malformed crown of a tooth with an artificial crown constructed of porcelain and/or gold. A well-fitting and properly contoured artificial crown will give many years of excellent service to its wearer. An improper fit, on the other hand, will result in premature failure and detachment of the artificial crown from the tooth.
The present invention is useful in preparing a well-fitting artificial crown. However, in order to understand the present invention it is helpful to first review the process involved in fitting an artificial crown, as well as several prior art types of retraction cord presently used in the state of the art.
FIG. 1 illustrates a normal tooth 20 requiring restoration by use of an artificial crown. Tooth 20 is surrounded by gingival tissue 22 and 24 (gingival tissue is commonly referred to as the "gums").
The dental practitioner first prepares the tooth to receive an artificial crown by removing portions of the tooth (shown by the broken line outline at 21) with a high-speed drill 26, as indicated in FIG. 2. The junction at which the uncut lower portion and the cut upper portion of the tooth meet is referred to as the "margin" 28. It has been found that in order to construct artificial crowns capable of remaining secured to the tooth in a physiologically sound condition for an extended period of time, it is critical to make an accurate impression in the area of margin 28. Failure to do so can result in decay around the area of the margin, or other physiological problems. Thus, premature failure of a crown is almost always due to poor marginal detail in the impression used to fabricate the crown.
Gingival supporting tissue 22, 24 is comprised largely of collagen fibers. Because these collagen fibers are highly elastic, the gingival cuff will begin to constrict around the margin 28 of the cut tooth almost immediately after the tooth has been shaped to receive a crown (see FIG. 3). This prevents adequate amounts of impression material from being placed around the margin 28 of the tooth 20. In addition, damage to the gingival tissue 22, 24 during cutting of the tooth 20 results in bleeding around the periphery of the tooth into the space formed between the gingival tissue 22, 24 and the tooth 20, this space generally being referred to in the art as the "gingival sulcus" 23, which is shown in FIGS. 2 and 3. This bleeding further interferes with the taking of a good impression because extravasated blood tends to prevent adequate cleaning and drying of the marginal area of the tooth prior to taking an impression and tends to displace the impression material before it can set.
Thus, the dual problems of contraction of the gingival cuff and the presence of hemorrhaging tissues make it impractical to simply take an impression following shaping of the tooth with the high-speed drill. Rather, four basic requirements have been identified for the taking of consistently acceptable impressions.
First, it is recognized that the gingival cuff must be horizontally retracted from around the marginal area of the tooth sufficient to permit an adequate bulk of impression material to be placed around the margin of the tooth. A second, and related requirement for the taking of an adequate impression is to displace the gingival tissue vertically in order to expose a portion of the tooth below the margin so that the impression will clearly indicate the location of the margin and will give good marginal detail. Third, all hemorrhage must be arrested prior to taking the impression, and fourth, the area where the impression is to be taken must be thoroughly cleaned and dried.
The most common method for providing horizontal and vertical retraction is by use of retraction cord. Thus, as illustrated in FIG. 4, a length of retraction cord 30 is packed between gingival tissue 22 and the margin 28 of tooth 20 by use of an appropriate dental instrument 32.
It is often necessary to pack several lengths of retraction cord into the sulcus 23 in order to obtain sufficient vertical and horizontal retraction of gingival tissue. FIG. 5 illustrates two possible approaches for utilizing multiple lengths of retraction cord. Thus, on the left side of tooth 20 it is indicated that three lengths of retraction cord 34, 36, and 38 might at times be used to obtain suitable gingival retraction. As better seen in FIG. 6, cord 34 causes vertical displacement of the gingival tissue in a direction toward the root of the tooth ("apical" displacement). This permits an accurate determination of the location of the margin 28. Cords 36 and 38 are then packed side-by-side over cord 34 and margin 28 so as to cause horizontal displacement of the gingival tissue. The result of the use of retraction cord is illustrated in FIG. 8; the margin 28 of the tooth is exposed sufficiently so that it is possible to make an impression having excellent marginal detail.
Returning to a consideration of FIG. 5, a second approach for retracting gingival tissue is illustrated on the right side of tooth 20. (It should be understood that in actual practice only a single approach would generally be taken with respect to a single tooth; the differences between the approaches taken on the right and left sides of tooth 20 in FIG. 5 are primarily for purposes of illustration.) Thus, in FIG. 5 it may be seen that it is possible to use only two retraction cords 40 and 42 to retract gingival tissue 24 in place of the three cords 34, 36 and 38 used in retraction of gingival tissue 22. Referring to FIG. 7, which is drawn to a larger scale, it may be seen that a small cord 40 corresponding to cord 34 of FIG. 6 can be used to accomplish vertical retraction, but a single cord 42 having a diameter approximately twice that of cord 40 may be used for obtaining the necessary horizontal retraction.
As mentioned above, gingival supporting tissue is comprised largely of flexible collagen fibers. Thus, although mere mechanical retraction is effective to some extent if the gingival tissue need not be displaced for more than a very short period of time after removal of the retraction cord, in practice it is usually preferable to apply a chemical retracting agent that reduces the tendency of the gingival tissue to contract over the margin after the retraction cord is removed. This is conveniently accomplished by soaking the retraction cord in a solution containing the chemical agent prior to packing the cord into the sulcus, or by pre-impregnating the cord with a suitable chemical agent. When such a suitable retracting agent and retraction cord are used, it is possible to maintain the gingival displacement illustrated in FIG. 8 for a period of time sufficient to permit the dental practitioner to make an impression.
As mentioned above, another requirement for the taking of a good quality impression is the arresting of any hemorrhaging caused by damage to gingival tissue during shaping of the tooth. Accordingly, in addition to use of a chemical retracting agent, it has been found useful to apply a chemical hemostatic agent to discontinue blood flow from damaged blood vessels. Again, this chemical agent can be applied by soaking the retraction cord in an appropriate solution prior to placement. Typically, the same chemical agent used to assist retraction also has hemostatic properties.
For many years, the only type of retraction cord available was constructed of multi-strand twisted cotton yarn. Although this type of retraction cord has been used extensively, it has a number of significant disadvantages. First, because only short lengths of cord are used, it is not uncommon for the twisted fibers to unravel. Also, there is a tendency for the dental packing instrument 37 (see FIG. 4) to slip between the twisted fibers, thereby making packing more difficult and often resulting in damage to underlying tissue. Further, withdrawing the dental instrument after it has slipped between fibers often results in pulling the cord from its position in the sulcus. Finally, because of the tendency of the twisted-strands to rearrange themselves, there is a tendency for this type of cord to flatten, thereby reducing its effectiveness in terms of horizontal or vertical retraction.
A substitute to the use of twisted cotton cord is disclosed in U.S. Pat. No. 4,321,038 wherein the use of a braided retraction cord is described. The weaving process used in the construction of this braided cord insures that the cord maintains its structural integrity so as not to unravel; a plurality of longitudinally disposed warp strands are tightly weaved together with a plurality of transversely disposed filling strands. Because of this structural integrity, there is no tendency for the dental instrument to slip between the various fibers used in construction of the cord and the cord will maintain its shape when packed into the gingival sulcus rather than flattening out in the manner of twisted-strand retraction cord.
However, notwithstanding these advantages, the braided cord also suffers from a number of disadvantages inherent in its design since the very properties that cause it to retain its shape also have undesirable side effects. For example, braided retraction cord has very little "give" (i.e., is not very resilient in either the longitudinal or transverse dimensions). It will be appreciated from a consideration of FIG. 4 that since the braided cord 28 prevents the tip of instrument 32 from slipping between the braided-strands (not shown), and since there is relatively little give in response to the radially or downwardly directed pressure exerted on the cord 30 by instrument 32, there is a tendency for the dental packing instrument 32 to slip off the cord 30 during packing. When this happens it is not uncommon for the tip of instrument 32 to impact the gingival tissue 22 or the base of the tooth 20 in a harmful manner.
Another disadvantage when utilizing a braided cord is that when the various fibers are weaved together closely enough and tightly enough to prevent a dental packing instrument from slipping between the fibers, the spaces between fibers are quite small and the cord has a rather small capacity for soaking up and retaining liquids. This limits the amount of liquid chemical hemostatic and/or retractive agent that can be carried into the sulcus by the braided retraction cord, which in turn reduces the effectiveness of the retraction cord in effecting retraction and in arresting hemorrhage.
Yet another disadvantage of the braided type cord is that it is not very elastic because of the tightly braided-strands. Thus, packed portions of the braided cord tend to become dislodged as the dental practitioner continues to pack additional portions of the cord around the margin of the tooth. This tendency to dislodge, which is commonly referred to as "popping out," is caused by variations (i.e., having to pack the cord around high and low points) that are generally present within the floor of the sulcus adjacent the margin of the tooth as one moves around the periphery of the margin. Because of the lack of elasticity in the longitudinal dimension of braided cord, when the dental practitioner places downward pressure on one portion of the cord, other portions of the cord are pulled out of place. In come cases the cord will actually "pop out" of its position in the sulcus.
One further disadvantage of braided retraction cord is encountered when packing the cord around anterior or front teeth where the gingival tissue is thinner and more easily damaged on the labial (lip) side of the tooth. For instance, a braided cord that will adequately retract the sulcus on the lingual (tongue) side, because of its lack of elasticity in the transverse dimension, may overstretch the sulcus on the lip side of the tooth, thereby causing unacceptable damage to the sensitive gingival tissues. Accordingly, use of braided cord in connection with the front teeth becomes very difficult and cumbersome as several cords must be packed into some portions of the sulcus, while only one or two cords are packed into other portions of the sulcus.
In view of the foregoing, it will be appreciated that it would be a significant advancement in the field of gingival tissue management preparatory to the taking of impressions if an improved retraction cord could be provided that successfully avoided the aforementioned problems.